1. Field of the Invention
The present invention is generally directed to inverter circuits and, more particularly, to improvements in such circuits which protect the power transistors from failure due to excessive junction temperature, current, voltage, or a combination of these.
2. Description of the Prior Art
Inverter circuits are commonly used to convert a d.c. voltage from batteries or a similar source to an a.c. voltage. The a.c. voltage may be stepped up or down and rectified or not depending on the load being supplied. Such circuits typically comprise an iron core power transformer having a center tapped primary winding and a secondary winding. A pair of switching transistors are connected to the primary winding and controlled by a driver circuit to alternately conduct to generate the a.c. current through the primary winding which is inductively coupled to the load attached across the secondary winding.
One common failure mode of these circuits occurs when the power transformer saturates causing very high currents in the power transistors. A number of approaches have been developed in the prior art to prevent power transistor failure. Representative of these are the following U.S. Patents.
U.S. Pat. No. 3,638,100 to Centala et al discloses a circuit to prevent the power transformer of an inverter circuit from presenting a low impedance upon core saturation. The Centala et al circuit employs an auxiliary commutating transformer whose windings are connected with those of the power transformer. The secondary windings of the auxiliary transformer are connected via diodes and an LC filter to a load. Upon saturation of the power transformer core, a high-reflected load impedance is presented to the power transistors.
U.S. Pat. No. 3,539,905 to Schwarz discloses a circuit which, like that of Centala et al, uses an auxiliary transformer. In this case, however, the auxiliary transformer directly measures the magnetising current by subtracting the load or secondary ampere-turns from the primary ampere-turns. When the magnetising current exceeds a given threshold (an indication of impending saturation), the power transistors are switched.
U.S. Pat. No. 4,162,524 to Jansson discloses a d.c. to d.c. converter which includes a circuit for sensing whether a current change in the main current path of the switching transistor reaches a given value subsequent to an initial current rise, if any, at the start of a conduction period. A control circuit is responsive to the sensing circuit to remove the drive to the switching transistor if the given value is reached.
U.S. Pat. No. 3,305,793 to Kammiller discloses a d.c. to a.c. converter with an overload protection circuit. This circuit includes a bypass current path paralleling a bistable voltage source connected to a load. Upon the occurrence of a relatively large load current surge, the bistable voltage source is in effect removed from the circuit.
U.S. Pat. No. 3,950,691 to Ohba discloses a d.c. to a.c. inverter with an overload protection circuit. This circuit senses potentially damaging transient signals, isolates those signals from the amplifier circuitry, and returns most of the power of the transient signals to the d.c. supply. Individual emitter resistors are used to control the maximum collector currents in the output transistors.
U.S. Pat. No. 3,351,840 to Yearley discloses a saturating-transformer type of inverter in which diodes and Zener diodes have been added to prevent switching transistor collector spikes.
U.S. Pat. No. 3,670,234 to Joyce discloses an overvoltage protector circuit connected between the regulated output and the midpoint of the primary side of a driver transformer in a pulse width modulated voltage regulator.
While the prior art circuits have provided a degree of protection to the power transistors in inverter circuits, there is a continuing need to improve that protection, particularly in inverter circuits which must exhibit high reliability in the field. Moreover, it is desirable to provide such protection to the switching transistors of inverter circuits in a simple and inexpensive way.